One of widely known conventional glow plugs used in internal combustion engines such as diesel engines is a glow plug equipped with a pressure sensor for detecting combustion pressure in an internal combustion engine. Such a glow plug typically includes a housing, and a heater unit protruding from the housing. For example, Patent Document 1 discloses a glow plug including a heater unit (finger) which is reduced in diameter stepwise such that the heater unit has a small-diameter front portion and a large-diameter rear portion. The heater unit of the glow plug described in Patent Document 1 is connected to a housing (body) through a membrane-like connecting member that allows the heater unit to move in an axial direction. A sensor disposed in the glow plug detects the axial displacement of the heater unit to thereby detect combustion pressure.
However, in the glow plug described in Patent Document 1, since the connecting member is connected to the large-diameter section of the heater unit, a limitation is imposed on the area of the connecting member. Therefore, the amount of axial displacement permitted for the heater unit is not sufficient, so that the accuracy in detecting combustion pressure may be low.